1. Field of the Invention
The present invention relates to a radio wave arrival direction estimating antenna apparatus for controlling a radiation pattern of an antenna used in a base station in a mobile communication system, mainly such as a portable telephone, a PHS and a pager.
2. Description of the Related Art
In a mobile communication field, as systems for accommodating a plurality of mobile stations in an area assigned for each base station, for example, Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and Code Division Multiple Access (CDMA) are practically used. A base station has a service area which is fixed with a predetermined size, and communicates mobile stations only existing in such an area. The frequency used by the base station and the mobile stations existing in the service area of the base station is also predetermined in such a manner that the interference does not occur in neighboring base stations and mutual mobile stations. Therefore, the number of mobile stations to be accommodated in an area of a base station is limited in the above-mentioned systems. Specifically, in the TDMA system, such a number is limited by the total number of communication slots assigned for mobile stations. In the FDMA system, such a number is limited by the total number of frequency channels. In the CDMA system, such a number is limited by an interference elimination capability determined by a ratio of a data transmission rate and a spread spectrum chip rate.
As a method for increasing an upper limit of the number of accommodated mobile stations, xe2x80x9cDynamic Zone Configurationxe2x80x9d is proposed (for example, xe2x80x9cEfficient Utilization of Radio Spectrum through Intelligent Technologyxe2x80x9d CRL frequency resource project presentation paper, October, 1996). In this technology, a radiation pattern of an antenna of a base station is transformed corresponding to the number of mobile stations existing around the base station and directions thereof, so that a shape of a radiation pattern area is transformed to a direction of a place where a large number of mobile stations exist. It is thereby possible to cancel an unbalance of communication traffic occurring in some region where a base station exists when the shape of the area is fixed, thus making it possible to increase the total number of terminals as mobile stations. Further, in this technology, it is possible to reduce interference in other mobile stations and base stations and to increase a communicable distance, by setting a beam of the radiation pattern to be narrow. In order to achieve such a technology, it is necessary to estimate the number of mobile stations and directions thereof by estimating arrival directions of radio waves from the mobile stations beforehand, and to direct a peak direction or a null direction of an antenna radiation pattern to the estimated direction. In a conventional technology, such a technique is achieved with a configuration composed of a radio wave arrival direction estimating section, a digital beam generating section and others, using a MUSIC method and an ESPRIT method as a radio wave arrival direction estimating technique, as described, for example, xe2x80x9cTrial Manufacture and Evaluation of Broadband Adaptive Array Antennaxe2x80x9d (Technical report of IEICE, AP97-76, 1997-07, Pages 39 to 44).
When the above-mentioned conventional technology is used, in the MUSIC method and the ESPRI method used as the radio wave arrival direction estimating technique, it is necessary to sample a large number of data to obtain the estimation, and to execute a large number of calculations, such as covariance matrix, eigen values, and eigen vectors, as the processing to perform the estimation.
Further, it is difficult to execute concurrent calculation processing, which is generally used to increase a processing speed, because the above-mentioned methods use a matrix calculation with a large number of elements. Therefore, there is a problem that the conventional methods are not applicable to the case where a mobile station is expected to pass though around a base station with a high speed.
An object of the present invention is to achieve a radio wave arrival direction estimating antenna apparatus for enabling the processing in a practical time for arrival direction estimation and antenna radiation pattern control with respect to a mobile station which moves around a base station with a high speed.
The object is achieved by a radio wave arrival direction estimating antenna apparatus having an array antenna with a plurality of antenna elements each for receiving a radio wave transmitted from a mobile station in a base station, a frequency converting section for frequency-converting a received RF frequency signal at each antenna element of the array antenna into a respective intermediate frequency signal, an analog-digital conversion section for converting the intermediate frequency signal into digital data, a downsampling section for sampling the digital data obtained in the analog-digital conversion section at a lower frequency, an arrival direction estimating section for estimating an arrival direction of the radio wave using digital data converted in the analog-digital conversion section, and an arrival direction tracking section for estimating a change of the arrival direction of the radio wave from the mobile station, using an estimated result by the arrival direction estimating section as an initial value and digital data sampled at a lower frequency by the downsampling section, to determine a direction sequentially.
According to the aforementioned configuration, the arrival direction estimating section estimates the number of mobile stations existing around the base station and positions thereof at certain intervals using the MUSIC method or the ESPRIT method. Using the estimated value as initial values, the arrival direction tracking section calculates a difference between a present direction where a mobile station exists and a previous direction obtained one sample before the present time, and estimates a direction of the mobile station using digital data. It is thereby possible to simplify the processing for estimating a direction of a mobile station, and to correctly estimate the number of mobile stations and directions thereof because the arrival direction estimating section estimates the number of mobile stations and directions thereof at certain intervals, thus enabling the judgment of a change of the number of mobile stations. Further, the arrival direction estimating section estimates an arrival direction of radio wave at a short interval, so that the arrival direction tracking section can track a moving of a mobile station adequately.